1. Field of the Invention
The invention relates to a toner for electrostatic image development to be used for an electrophotographic image formation apparatus and a process for preparing the same Furthermore, the present invention relates to an electrostatic image developer, an image formation method using the developer, and a formed image.
2. Description of the Related Art
Along with the remarkable advancement of recent OA appliances, copying machines, printers, and facsimile apparatuses for carrying out printing by utilizing an electrophotographic method have become popular. An electrophotographic image formation apparatus generally carries out image formation by charging, exposing, developing, transferring, cleaning, and fixing steps.
In the charging step, a surface of a photoconductor is evenly charged in a dark place. In the exposing step, an original image is projected to the charged photoconductor to form an electrostatic latent image on the surface of the photoconductor by removing the electric charge in the portion where light is radiated. In the developing step, a toner image (a visible image) is formed by binding a toner to the electrostatic latent image formed on the surface of the photoconductor. In the transferring step, the toner image formed in the surface of the photoconductor is transferred to a recording medium such as paper and a sheet by bringing the toner image formed in the surface of the photoconductor and the recording medium into contact with each other, causing corona electric discharge from the opposed side to the face of the recording medium contacting the toner image, and applying the electric charge with the opposed polarity to that of the toner to the recording medium. In the fixing step, the toner image is fixed to the recording medium by heating or pressurizing means. In the cleaning step, the toner not transferred to the recording medium and remaining on the surface of the photoconductor is recovered. An electrophotographic image formation apparatus forms a desired image on a recording medium through the above-mentioned steps.
A process for preparing of the toner for electrostatic image development to be used an electrophotographic image formation apparatus may include a dry method and a wet method. A pulverization method can be exemplified as the dry method. The pulverization method is a method of obtaining a toner by melting and kneading a resin, a pigment (a coloring agent) and a wax, and pulverizing and classifying the obtained kneaded product by a pulverizer and has been employed widely in an industrial field. A toner is desired to have a small particle diameter so as to form high quality images, however the pulverization method takes increased quantities of energy and time for pulverization and the process becomes complicated and lowers the yield and thus there has been a problem that the product cost considerably soars.
A suspension polymerization method and an emulsion polymerization method can be exemplified as the wet method. The suspension polymerization method is a method of obtaining a toner by suspension-polymerizing synthetic resin monomers such as vinyl monomers in an aqueous solvent containing a pigment.
On the other hand, the emulsion polymerization method is a method of obtaining a toner by mixing an aqueous dispersion of synthetic resin particles and a dispersion of a pigment dispersed in an organic solvent for forming agglomerated particles of the synthetic resin particles and the pigment and heating and melting the agglomerated particles. As it is described in later, this method uses a surfactant in a resin particle dispersion of starting substance for toner preparation and therefore a surfactant remains in the toner to be produced.
There is a method using phase inversion emulsification as another wet method. The phase transfer emulsification method is a method of obtaining toner particles by dissolving or dispersing self-water dispersible resin and pigment in an organic solvent and adding water while adding and stirring a neutralization agent for neutralizing a dissociation group of the resin and thereby carrying out phase transfer emulsification of the resin solution droplets enclosing the pigment. Also, there is another wet method of obtaining toner particles by dissolving or dispersing a resin in a solvent in which the resin is soluble, granulating the obtained solution or dispersion in a water medium containing an inorganic dispersant, removing the solvent, and drying the formed particles. This process of a toner for electrostatic image development is for producing a toner by dissolving or dispersing materials including a polyester resin and a pigment in a solvent in which the polyester resin is soluble, granulating the obtained solution in an aqueous medium containing an inorganic dispersant, and then removing the solvent.
The above-mentioned preparation of the toner by the wet method leaves the organic solvent, monomers of the resin, and a surfactant in the toner to be obtained. Such components bleed out of the toner at the time of practical used of the toner for development of an electrostatic latent image and cause damages of parts such as a development roller. Further, the chargeability of the toner becomes uneven. Accordingly, a step of removing the organic solvent, the monomers, and the surfactant from the toner is required after the toner is produced. However, the shape and the chargeability of the toner to be obtained tend to become uneven in accordance with slight fluctuation of operation conditions relevant to the pressure, temperature, and time in such a removal step. Therefore, to obtain a toner with a uniform shape, it is required to adjust the operation conditions to be optimum and it is very difficult to accomplish the adjustment. Further, since the organic solvent, the monomers, and the surfactant, which are heavy loads on ambient environments, are used in large quantities, the facilities to treat them are required to lead to the cost up of toner preparation. Further, in form of the toner, monomers remain in this emulsion polymerization method. Therefore, there occurs a new problem that remaining monomers are evaporated and emit malodor when heat and pressure are applied to the toner for fixation or the like.
Further, the toner to be obtained by the above-mentioned conventional wet method is not necessarily excellent all in the toner properties such as the image density and the image quality such as fogging, of the image to be formed on a recording medium by using it as well as the dispersibility of the pigment on the recording medium and the transfer ratio of the toner to the recording medium.
For that, as another conventional wet method, Japanese Patent Laid-Open Application No. 10-39545 (Document No. 1) discloses a toner process neither using any organic solvent nor involving any polymerization reaction at the time of forming toner particles. In the process of a toner composition of the Document No. 1, adding an aqueous pigment dispersion obtained by dispersing a pigment in water by using a sodium sulfonated polyester to an emulsion obtained by dispersing the sodium sulfonated polyester in water, further adding an alkali halide solution, and thereby forming agglomerates.
Further, Japanese Patent Laid-Open Application No. 2002-131977 (Document No. 2) discloses a toner process similarly using a self-dispersible resin containing a hydrophilic ethylenic unsaturated monomer. In the Document No. 2, sodium sulfonate or potassium sulfonate of a styrene derivative is used as the hydrophilic ethylenic unsaturated monomer. The method of the Document No. 2 is a method of producing a toner using the self-dispersible resin containing a hydrophilic group of a sulfonic acid metal salt, which is a polyvalent metal salt, as a flocculant.
Also, as a patent using a self-dispersible polyester is exemplified Japanese Patent Laid-Open Application No. 2004-354411 (Document No. 3) and No. 2005-140855 (Document No. 4). They disclose a process of a toner using a mixture of a vinyl type resin dispersion and a self-dispersible polyester. In those documents, it is disclosed that the self-dispersible polyester is preferably polyester resins containing no trivalent monomer. Also, it is disclosed that an ionic surfactant is used as a flocculant for causing soft flocculation.
A toner for electrostatic image development is desirable to contain no unnecessary components such as an organic solvent or monomers, to be produced easily, and to have excellent toner properties.
With respect to the toner process of the Document No. 1, a toner composition is produced by flocculating a pigment dispersed in water by using a sodium sulfonated polyester and a sodium sulfonated polyester dispersed in water and thus the toner composition process capable of producing a toner composition free from unnecessary components such as an organic solvent and monomers. However, since the dispersibility of the pigment is insufficient if the pigment is dispersed by using only the sodium sulfonate polyester, aggregates of solely the pigment are formed the toner property such as a color reproducibility cannot be satisfied and thus the process is unsuitable for producing a toner composition having excellent toner properties. Further, the toner composition produced by the method becomes unstable in the chargeability depending on the ambient environmental conditions and it has a problem that its use is limited in accordance with the environmental conditions and it is supposed that the problem is attributed to the sulfonic acid group, a hydrophilic group.
Similarly, also in the toner process disclosed in the Document No. 2, although the pigment dispersibility is maintained, the properties of a toner to be produced in relation to the ambient environmental conditions have similar problems because of the above-mentioned same reason since the hydrophilic group is a sulfone metal salt.
Further, with respect to toners respectively disclosed in the Document No. 3 and No. 4, they are produced using the mixture of the self-dispersible polyester and the vinyl type resin dispersion. Though the vinyl type resin dispersion is used as mentioned above, the toners contain a monomer. Also, since soft flocculation is caused by using a surfactant, it is very difficult to control the particle diameter.